Multiple heat range spark plug

ABSTRACT

A spark plug incorporating a heat shunt on the plug insulator nose arranged such that the characteristics of a &#39;&#39;&#39;&#39;hot&#39;&#39;&#39;&#39; plug are provided at lower operating temperatures and of a &#39;&#39;&#39;&#39;cold&#39;&#39;&#39;&#39; plug at higher operating temperatures. The heat shunt is a thermally conductive ring bonded on the insulator nose. A thermal gap between the heat shunt and the shell of the plug prevents heat transfer through the shunt at lower operating temperatures-there is thus a relatively long heat path into the cooling system and the firing end of the plug stays relatively hot to prevent fouling. At higher operating temperatures, the heat shunt expands thermally to bridge the gap to thereby shorten the heat path into the cooling system so that the relatively rapid heat conduction &#39;&#39;&#39;&#39;cools&#39;&#39;&#39;&#39; the firing end of the plug to prevent its overheating under high speeds and loads. A method for fabricating the spark plug is given in which the plug insulator with the shunt bonded thereon is installed in the shell at the temperature at which the shunt is designed to have expanded into good thermal contact with the shell whereby the heat shunt gap is set substantially automatically. A method is also provided for monitoring the setting of the shunt gap during the manufacturing of the plug whereby a correlation is made between an energy level and thermal conduction in the shunting means.

United States Patent [72] Inventor William P. Strumbos 85 Middleville Road, Northport, N.Y. 11768 [21] Appl. No. 18,615

[22] Filed Mar. 11, 1970 [45] Patented Oct. 12, 1971 [54] MULTIPLE HEAT RANGE SPARK PLUG 11 Claims, 12 Drawing Figs.

[52] U.S.Cl 313/11.5, 29/25.12,313/118,313/143 [51] Int. Cl H01t 13/16 [50] Field of Search 29/2512; 3l3/11.5,1l8,119,130,143

[56] References Cited UNITED STATES PATENTS 2,212,725 8/1940 Andtes 313/11.5 X

2,831,993 4/1958 Lentz 313/1 1.5

3,130,338 4/1964 Andersen 313/] 1.5

FORElGN PATENTS 5,467 12/1932 Australia 313/115 Primary Examiner-Roy Lake Assistant Examiner-E. R. LaRoche ABSTRACT: A spark plug incorporating a heat shunt on the plug insulator nose arranged such that the characteristics of a hot" plug are provided at lower operating temperatures and of a cold plug at higher operating temperatures. The heat shunt is a thermally conductive ring bonded on the insulator nose. A thermal gap between the heat shunt and the shell of the plug prevents heat transfer through the shunt at lower operating temperatures-there is thus a relatively long heat path into the cooling system and the firing end of the plug stays relatively hot to prevent fouling. At higher operating temperatures, the heat shunt expands thermally to bridge the gap to thereby shorten the heat path into the cooling system so that the relatively rapid heat conduction cools" the firing end of the plug to prevent its overheating under high speeds and loads. A method for fabricating the spark plug is given in which the plug insulator with the shunt bonded thereon is installed in the shell at the temperature at which the shunt is designed to have expanded into good thermal contact with the shell whereby the heat shunt gap is set substantially automatically. A method is also provided for monitoring the setting of the shunt gap during the manufacturing of the plug whereby a correlation is made between an energy level and thermal conduetion in the shunting means.

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PATENTEUnm 12 l97l SHEET 2 0F 2 INVENTOR.

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MULTIPLE HEAT RANGE SPARK PLUG BACKGROUND OF THE INVENTION This invention relates to spark plugs for internal combustion engines and, more particularly, to a spark plug which is provided with means to vary the heat range of the plug automatically.

Spark plugs, particularly those in high-speed, high-compression engines, are subjected to an extreme range of pressure and temperature conditions. Plug temperatures range from about 400 F. at low engine speeds and light loads, to as high as l,600 F. under full throttle, full load. Below about 840 F., carbon and other products of combustion begin to form on the plug insulator nose. If not removed, those deposits build up until current shorts through the deposits instead of sparking across the electrodes. At normal speeds, enough heat is usually generated to burn those deposits away as quickly as they are formed. However, when high speeds or heavy loads raise the plug temperatures above l,lO F. to l,300 F., the deposits, particularly those resulting from the additives in currently available fuels and lubricants, are melted to form a glaze coating on the plug insulator nose. When hot, this glaze is highly conductive and the plug is shorted out. This causes misfiring with consequent fuel and power losses. Should plug temperatures become excessive, the plug points themselves become hot enough to ignite the fuel-air mixture in the cylinder. This causes autoignition and, if continued, can lead to the destruction of the plug and serious engine damage. Overheated electrodes also cause a condition commonly met in two-stroke engines: the bridging of the electrodes due to the buildup of conducting deposits formed by combustion particles which have melted upon their striking the overheated electrodes. in plug temperature ranges above 1,600 F., chemical corrosion and spark erosion cause plug failure within a very short time.

it will be seen then, if a hot-type plug is subjected to high compression pressures, temperatures, and loads, electrode burning and autoignition will result because of the plugs slow rate of heat transfer. A cold plug, because it will not reach full operating temperature, will not tolerate low speed, light-load operation for any length of time without becoming fouled with current-conducting deposits. Because a cold plug under such conditions will not reach a temperature required to burn off fouling, carbon formation as well as additive particles from the fuel and oil will condense on the comparatively cool surfaces of the insulator to foul the plug and to cause it to misfire.

Spark plugs are customarily supplied in various heat ranges to handle the requirements of individual engines and operating conditions. Heat range refers to the ability of the plug to conduct the heat of combustion away from the electrodes or firing end. As shown in FIG. 1, a hot-type plug will have a long insulator nose 2. Because of the length of the heat path (as indicated by the arrows 3), heat thus will be transferred comparatively slowly from the plug firing end to the engine cooling system. A coldtype plug (FIG. 2), on the other hand, has a comparatively short insulator nose 4 and heat is transferred rapidly (as indicated by the arrows 5) into the engines cooling system.

The prior art discloses several examples of spark plugs incorporating means which are intended to vary the heat range automatically such that the plug can accommodate a wider spectrum of operating conditions. In one such example in the prior art as disclosed by P. G. Andres in US. Pat. No. 2,212,725, a skirt having fingers of bimetallic material is positioned on the ceramic insulator such that the fingers contract into close contact with the insulator to provide a highly conductive thermal path therefrom upon the plug reaching some predetermined temperature. Inasmuch as the conduction of heat from an object is dependent upon the establishment of a good thermal contact, and the design in the cited Andres patent is such that any appreciable plug fouling will adversely effect the thermal contact, the optimum performance characteristics of the devicemay be erratic and dif-- ficult to maintain under all conditions of operation. In another such spark plug disclosed by H. W. Andersen in U.S. Pat. No. 3,130,338, a substantially similar skirt of bimetallic material is fitted on the plug ceramic insulator in the area above the lower insulator gasket of the plug. Although that Andersen design will have some effect on the overall plug temperatures, it does not appear that it would effect to any appreciable extent the temperature of the nose or firing end of the plug which is the area of concern of the instant invention.

SUMMARY OF THE INVENTION Therefore, to overcome the foregoing and other difficulties of the prior art, the general object of this invention is to provide means for varying the heat range of a spark plug automatically to thus keep the plug at the most efiective temperature during all operating conditions such that starting, warmup, idling, lowand high-speed operation of the engine is improved. And, further, to accompany such improvement in engine performance with an efficient spark plug design that reduces the causes of misfiring so that the engine produces greater power and increased fuel economy in all speed ranges.

It is another object of this invention to provide a multiple heat range spark plug whose operating temperature is automatically varied such that the plug runs hot at the lower cylinder temperatures occurring when the engine is idling or at low speeds and loads to thereby inhibit plug fouling, and which runs relatively cool at higher cylinder temperatures such as those occurring under conditions of high speeds and loads so as to prevent the plug overheating that causes autoignition and plug electrode burning.

Another object is to provide a spark plug whose design eliminates the requirement for a specific heat range in a plug so that the number of spark plug types required to be manufactured or that have to be stocked by the dealer are thereby reduced. A concomitant object is to provide a spark plug having a multiple heat range such that the selection of a plug with the proper heat range for a specific engine or for the type of service that the engine will encounter will no longer be a problem such that the possibility of fitting plugs of the wrong heat range is an engine with the attendant probability of poor performance and engine damage or owner dissatisfaction is thereby avoided.

A further object is to provide a method using low-cost production processes that will allow a high standard of precision to be attained in the manufacture of a spark plug of the above type fitted with a heat shunt having a precisely sized shunt gap for varying automatically the heat range of the plug.

A still further object is to provide a method adaptable for use in low-cost, high-volume production processes for monitoring the setting of the thermal gap in a spark plug provided with a heat shunt, which method uses a correlation between a readily instrumented energy level and heat conduction in the shunting means such that a high standard of quality control is feasible in a production shop.

Yet another object is to provide a spark plug having automatic means for varying the heat range such that an optimum operating temperature is maintained to thereby minimize the plug fouling that leads to the misfin'ng which results in engine emissions that contribute heavily to environmental air pollution. in addition, it is an object to provide a plug that will maintain a high standard of performance with engine fuels that have their volatility reduced and have some of their additives and compounds eliminated as a pollution curb.

Other objects and advantages, and a more complete understanding of the invention, will become apparent from the following description and claims, taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS For the purpose of illustrating the invention, there is shown in the drawings the forms which are presently preferred, it being understood, however, that this invention is not necessarily limited to the precise arrangements and instrumentalities here shown.

FIG. I is a side elevation in partial longitudinal section of a prior art spark plug of the hot type in its operating environment in an engine cylinder head;

FIG. 2 is a similar view of a prior art spark plug of the cold type;

FIG. 3 is a side elevation in partial longitudinal section of a spark plug embodying the heat shunting means of the invention;

FIG. 4a is a side elevation in partial section of the left half of the spark plug of FIG. 3 in place in the cylinder head of an en gine and showing the heat path in the hot range of the inventlon;

FIG. db is a similar view of the right half of the spark plug of FIG. 4a showing the heat path in the cold range of the invention;

FIG. 5 is a partial sectional view of the lower end of a plug showing a further embodiment of the heat shunting means of the invention;

FIG. 6 is a side elevation in partial longitudinal section of a spark plug showing a further embodiment of the invention;

FIG. 7 is a partial sectional view of the lower end of the spark plug of FIG. 3;

FIG. 8 is a partial sectional view with portions broken away of an element of the heat shunting means of the plug of FIG. 3;

FIG. 9 is a partial sectional view of the lower end of a plug showing a further embodiment of the invention;

FIGS. 10 and l 1 are partial sectional views of the lower end of a spark plug of the invention and showing diagrammatically equipment used to monitor the adjustment of the heat shunting means; and

FIG. 12 is a partial sectional view with portions broken away of shielding means for the heat shunting means of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 3 of thy drawings, there is shown a spark plug 10 comprising an insulator 11 positioned within a metal shell 12 in a gastight relationship therewith. The insulator 11 is formed of alumina or other suitable material in the conventional manner with an upper shoulder 13 and a lower shoulder 14. A sealing gasket 15 is positioned on upper shoulder 13 and a sealing gasket 16 is provided between the lower shoulder M and an annular ramped portion or ledge 17 formed on the inner surface of the shell 12. The insulator 11 has a body portion 18 and a tapered tip or nose portion 19 and is provided with a centerbore within which a conventional center electrode assembly 20 is secured. Center electrode 20 protrudes from the lower end of the insulator ll 1 and the electrode lower end constitutes a firing tip which forms a spark gap with a ground electrode 21 which is suitably attached as by welding to the metal shell 12. The shell 12 has a shoulder 22 below which is a threaded portion 23 formed to engage in a well-known manner in a threaded bore 24 in the cylinder head 25 of an engine 26. (See FIG. 4.) A gasket (not shown) may be provided if required as a sealing means between shoulder 22 and cylinder head 25. The cylinder head may be provided with cooling means, typically passages 27 through which is circulated a coolant such as fluid 28 for cooling the engine. It will be appreciated that details of the engine are given merely for ex ository purposes only and it forms no part of the invention.

It will also be appreciated that the spark plug as described above is substantially a conventional plug fabricated with wellknown materials using suitable known techniques. The instant invention resides in the provision of heat shunting means on the nose portion 119 of the insulator 11. The design of the heat shunt is susceptible to many variations as to construction and design temperature as will be described below in greater detail, but in a basic form may simply comprise a metal ring 101 bonded on the nose portion 119 of the insulator 111 of the spark plug IEO as shown in FIG. 5. Shunt ml. can be bonded permanently in place by a hard-soldering technique after the faying surface of the insulator nose portion 119 has been prepared properly as by a metallizing process as is well known in the art. Between the outer peripheral face 102 of the heat shunt and the inside wall surface 103 of the spark plug shell 112 (when the plug is below its design temperature of about 900 F.) is an annular gap 104. Gap 104 is sized with respect to the coefficient of thermal expansion of the heat shunt 101 such that at the design temperature the shunt will have expanded into good heat transfer relationship with wall surface I03 of the shell. When the shunt has thus expanded into thermal contact with the shell, heat will be conducted rapidly away from the firing end of the plug, passing by means of the shunt 101 directly into the plug shell 112 and thence into the cooling system of the engine (not shown).

Good design practice dictates that the heat shunt be fabricated from a material (or materials, if a composite or a bimetallic construction is used) of high thermal conductivity bonded in good thermal contact on the insulator nose. The shunt is located most suitably in a position on the insulator nose that corresponds with the location of the lower sealing gasket 6 (see FIG. 2) in a spark plug of the conventional cold type. The distance between the shunt and the bottom edge of the insulator nose in the spark plug of this invention thus corresponds with the distance between the lower sealing gasket and the bottom edge of the insulator nose in the conventional cold-type plug. (This location is not intended as a limitation, of course, and the shunt may be positioned in any preferred location to serve the requirements of any specific application.) The outer peripheral face of the heat shunt is spaced a predetermined clearance from the inner bore of the metal shell of the plug. The size of the gap between the shunt and the shell bore is determined by the coefficient of thermal expansion of the heat shunt (compensation being made, of course, for the thermal growth of the shell itself) and is calculated such that the expansion of the shunt will close the gap and establish good thermal contact with the shell at some predetermined plug temperature. To avoid the plug fouling that occurs at plug temperatures below about 900 F., the heat shunt should be designed to become effective at about that temperature, preferably somewhere in the range between 900 F. When the heat shunt gap closes, heat will be conducted rapidly away from the firing end of the plug in a manner analogous to a cold-type plug to allow high cylinder temperatures to be accommodated without damage or loss of performance. In the following specification, the temperature at which the thermal expansion of the heat shunt has caused it to expand into good thermal transfer contact with the spark plug shell will be referred to as the design temperature; this temperature will preferably be in the 900l,l00 F. range, but this range is not intended to be a limitation and can be changed to suit the requirements.

It will be understood that the gap referred to is a condition in which heat is not effectively conducted between the shunt and the wall of the plug shell and an actual perceptible gap may not really exist. Because good heat transfer is a function of the thermal contact between the elements involved, the elements can actually touch one another without the establishment of the required contact that would allow heat transfer. Thus, it will be understood that the term gap" herein refers to a condition in which heat transfer is substantially obviated even though the elements may be in what is commonly regarded as physical contact. There is an advantage in the selection of a material having a low coefficient of thermal expansion for the heat shunt in that, with such material, the shunt may be in, or almost in, contact with the shell wall even below the design temperature so that the possibility of the buildup of combustion products between the two elements that would degrade performance is thereby avoided. However, as will be described in greater detail elsewhere in this specification, shielding means can be provided for the heat shunt to prevent a contamination of the gap;

To prevent damage to the insulator, principally, or to the shunt bond or the other elements of the plug due to a thermal overexpansion of the heat shunting means, design considerations dictate a measure of flexibility therein. To this end, the shunt may comprise a hollow shunt ring or may be slotted in any appropriate way to give it the required resiliency so that the shunt deflects or gives" in the event of thermal overexpansion to thus avoid damage.

A further embodiment of a spark plug incorporating the heat shunting means of my invention is illustrated in FIG. 6. In this embodiment also, the heat shunt comprises a metal ring bonded on the nose portion 219 of the insulator 211 of the spark plug 210. However, unlike the heat shunt of FIG. 5, heat shunt 201 of this embodiment has a tapered outer peripheral face 205. A matching taper 206 is formed in the inside wall surface 203 of the spark plug shell 212. Between the tapered face 205 of the shunt 201 and the tapered wall portion 206 of metal shell 2K2 is the heat shunt gap 204. As previously discussed, gap 204 is sized with respect to the coefficient of thermal expansion of the heat shunt 201 such that at the design temperature the shunt will have expanded into good heat transfer relationship with wall surface 206 of the shell to thereby conduct heat rapidly away from the firing end 220 of the plug.

in the fabrication of the embodiment of the invention shown in FlG. 6; the shell 212; the insulator 2111 with its center electrode assembly; and the other elements such as the sealing gaskets 215 and 216; are of the conventional type formed in the usual manner with suitable well-known materials. The insulator 2M and shell 212 are of the hot-type plug design having a long insulator nose similar to that of the spark plug illustrated in FIG. 1. The heat shunt 201 is bonded on the insulator nose 21W at a location preferably the same as that conventionally occupied by the lower sealing gasket in a cold-type plug similar to that illustrated in FIG. 2. Most conveniently, the heat shunt is bonded in place prior to the installation of the center electrode assembly in the insulator. The taper 206 will be formed in any suitable way in bore 203 of the shell at any convenient time in the course of the production of that part.

To assemble the spark plug 210, the insulator 211 with sealing gaskets 215 and 21.6 are inserted into the bore 203 of shell 212. The assembly is then heated as by induction heating to bring it to the design temperature. After the assembly has stabilized at that temperature, pressure is applied to seat the insulator in the shell with the tapered face 205 of the shunt in good thermal contact with the tapered wall portion 206 of the shell (it will be appreciated that sealing gasket 216 will be of suitable resiliency to accommodate for the axial movement necessary to establish the proper shunt contact and also to compensate for dimensional variations due to manufacturing tolerances). Insulator 211 is then locked in place in the shell, preferably by spinning or swagging upper edge 240 of shell 212 down in a gastight relationship over sealing gasket 2E5 in a well-known manner. When the plug cools, the heat shunt will contract to establish the gap 204 (shown in exaggerated scale in the drawings to facilitate the description of the invention). It will be appreciated that this technique using a correlation of the coefficient of thermal expansion and a. predetermined temperature at which an element will have expanded a certain distance can be utilized to set the spacing between any other of the spark plug components having or which may be provided with matching tapered faces requiring a precise gap therebetween.

In spark plug 210 of FIG. 6, a taper 206 to match the tapered face 205 of the heat shunt is machined or otherwise formed in the inside wall surface 203 of the plug shell itself. In the preferred embodiment of my invention as shown particularly in FIGS. 3, 4, and 7, however, the tapered area of the shell is a tapered cylindrical insert 29 welded in place in the bottom portion 30 of the shell. This embodiment can best be described with reference to FIG. 7; as shown, the heat shunting elements of plug 110 will thus comprise a heat shunt 31 bonded in a heat-conducting relationship on the nose portion 19 of the insulator II and the insert 29 which is bonded in a heat-conducting relationship on the inner wall surface 33 of the shell l2. I-Ieat shunt 31 has a tapered outer peripheral face 35 which matches the taper 36 in the upper inside surface 32 of the insert 29 (FIG. 8). Insert 29 preferably is fabricated of the same material, for example, steel, as the shell 12 and comprises a base portion 37 and the tapered upper portion 32. An annular portion 38 of the upper outer wall of the insert is cut away or relieved such as to provide a measure of resiliency to the tapered portion of the insert to thus accommodate without damage any overexpansion of the heat shunt. Should greater resiliency be required, the tapered portion 32 of the insert can be provided with a series of radial cuts 39, or can incorporate other design expedients such as an undercut (not shown). In fabricating the embodiment shown in FIG. 3, an insulator 11 having a long nose portion 19 substantially the same as a conventional hot-type plug is used and the shell 12 and associated elements employed therewith will be dimensioned accordingly. The heat shunt 31 is bonded on the insulator nose 19: this bonding step may be done at any suitable time during the fabrication of the insulator. As discussed previously, the heat shunt will be bonded on the insulator nose in a position corresponding to the location of the lower sealing gasket in a conventional cold-type plug. Insulator 11 (complete with the heat shunt 31 and the elements comprising the center electrode assembly 20) with the sealing gaskets 15 and 16 are inserted in the bore of the shell 12 and the insulator is locked into the shell in a gastight relationship in any suitable way as by swagging the upper edge 40 of the shell 12 down securely over sealing gasket 15 all in a well-known manner. (The assembly procedure to this point will differ, however, from the conventional process in that the ground electrode 21 is not installed on the shell 12 as is often the case at this stage of manufacture.) The insert 29 is started in the lower end of the bore of the shell (the base portion 37 of the insert 29 preferably is an interference fit with the bore of the shell so as to insure proper heat transfer, but the relieved portion 38 has an appreciable clearance fit; of course, provisions can be made to cut threads into the outside diameter of the insert 29 and into the bore of the shell 12 such that the insert can be screwed into the bore, but because of the poorer heat transfer characteristics, such arrangement is not preferred) and the assembly is brought to the design temperature and is stabilized there by suitable means such as induction heaters or quartz lamps. Pressure is then applied to the bottom edge of the insert to force it axially into the bore of the shell so as to bring the tapered portion 36 of the insert into a good thermal transfer contact with the tapered face 35 of the heat shunt. When proper contact pressure has been established, the insert is bonded in place in any suitable manner such as by resistance welding, care being taken to insure that the insert is in good thermal transfer relationship with the shell to which it is bonded. At this time, the ground electrode 21 can be welded in place and, after the assembly has been allowed to cool, the spark gap between the ground electrode and the center electrode 20 can be set. When the assembled spark plug 10 has cooled, of course, the contraction of the heat shunt 31 will have established the proper gap 44 between the shunt and the insert.

As discussed previously, in operation with the spark plug installed in the engine, at plug temperatures below the 900 F. to l,l00 F. range, the gap 44 will prevent heat transfer through the heat shunt 31, thus the heat path will be substantially as shown by directional arrows 45 in FIG. 4a. As shown, heat will have to travel up the long insulator nose l9 and pass by means of sealing gasket 16 into shell 12 and thence into the cooling system in cylinder head 25 of the engine. Because of this relatively long heat path and because of the poor thermal conductivity of the material from which the insulator is formed, heat is conducted away from the firing end of the plug slowly such that the insulator nose heats up to a temperature high enough to resist deposits of oil and carbon even during prolonged periods of idle or low-speed operation. When the engine is running at high speeds or under load, the plug temperatures rise rapidly, causing the thermal expansion of the heat shunt. Upon reaching the design temperature, the expansion of the heat shunt will have been sufficient to have closed the gap and to have brought the shunt into thermal contact with the shell (the insert 29, of course, being considered an integral part of the shell). The heat path thus becomes that substantially as shown by directional arrows 46 in FIG. 4b. As shown, heat will pass from the firing end of the plug directly through the heat shunt into the shell, which is of metal and having good thermal conductivity, and then into the cooling system in cylinder head 25 of the engine. Thus, although there will be a rise in plug temperatures because of the higher cylinder temperatures, the heat shunt transfers heat rapidly into the engines cooling system to thereby avoid plug overheating and possible preignition and damage. (To facilitate the explanation of the invention, the heat paths of the plug in the two heat ranges are illustrated in opposite halves of the drawing of a single plug so that the paths may be more readily compared. it will be obvious, of course, that the heat flow in the plug in operation will be substantially symmetrical about its longitudinal axis and thus the path in any one heat range will be either one or the other of the two shown.)

It is thus seen that the heat shunt adds a desirable measure of control to the spark-plug operating temperatures and the contact area and material of the shunt, and other parameters can be varied to suit the requirements. The heat shunting means can also comprise more than one heat shunt or ring and the invention can be embodied in a design having two heat shunt rings (as shown in FIG. 9) or more. As shown, spark plug 10, is provided with two heat shunt rings 31, and 31 and two inserts 29 and 29 designed to accommodate the two rings. Except for the provision of two rings and the associated two inserts which give the plug two design temperatures, with the consequent necessity for varying the installation and gap setting procedures given above for a one shunt ring design, the components of the device are substantially identical to the embodiments set forth previously with any differences being sufficiently obvious to avoid any confusion: thus it is not believed that any further or detailed description of this embodiment would serve any useful purpose.

It will be appreciated that a good thermal-transfer relationship between the heat shunt and shell will be a function substantially of the shunt means area of contact and the contact pressure. The size of the contact area is determined by the size of the shunt and the plug designer can readily satisfy this requirement. Contact pressure in this design in operation is a function substantially of the proper installation of the insert during the fabrication of the spark plug. To insure that proper contact pressure is attained when the insert is installed, it is desirable to employ means during the manufacturing process to monitor the installation operation. The insert installation operation may be controlled by any suitable monitoring means, such as by ultrasonic or electrical equipment. The monitoring equipment is calibrated by running empirical tests to establish the level of contact pressure needed for the proper thermal transfer and the equipment is then set to indicate when this value has been attained. With ultrasonic monitoring equipment, this indication as to when proper contact pressure has been established will be determined by some energy level of a signal transmitted by a suitable ultrasonic emitter 50 and received by a receiver 51. As shown in FIG. 10, the signal is applied by the emitter 50 to the tip of the insulator l1 and passes into the shunt 31 and thence by means of the interface 52 between the face of the shunt and insert into the insert and then to the energy receiver 51. The received signal is processed by suitable known equipment 53, which produces an indication of the energy level, as by an indicator dial 54. With an electrical-type monitor (FIG. 11), an electrical signal from one pole of the monitor is passed through a probe 60 into the shunt 3H and thence by means of the interface 62 between the faces of the shunt and insert into the insert and completes the circuit by means of probe 61 connected into the monitor .63 where it is processed in a known way to produce an indication of some electrical characteristic of the monitoring signal. This characteristic may be resistance and may represent the resistance introduced into the monitoring circuit by the interface 62 between the shunt and insert. A certain resistance, as determined, for example, by a known ammeter 64/voltmeter 65-type measuring circuit, will indicate that the desired contact pressure has been attained. It is also feasible to use thermometric means to determine when the desired thermal contact is obtained. Suitable heating means (not shown) can be used to apply a heat input into the tip of the insulator 11 and temperature sensing means (not shown) such as thermocouples applied to the insert 29 can be employed to determine by means of the heat flow through the shunt into the insert (and shell) when proper contact pressure is established. To enable the monitoring of the assembly procedure to be conducted without interference with the production process, the active sensing elements of the monitoring equipment can be integrated into the assembly tools themselves. Thus, the tool used, for instance, to press the insert into place in the shell bore can have the sensor built right into the face of the tool engaged against the bottom of the insert.

Although the emphasis in the foregoing specification is on what can be considered a standard spark plug having a single electrode, the invention is not to be construed as being limited to such type and the scope of the invention embraces the other types such as the multiple-electrode and surface-discharge spark plugs also. In addition, the heat shunt set forth is of an annular form, but it will be recognized that other configurations having segmented pieces and the like are encompassed within the meaning of the invention.

To insure the proper operation of the spark plug of this invention, the emphasis in the design should be such that the buildup of foreign matter and combustion products on the surfaces of the heat shunt and shell facing on the shunt gap is inhibited or prevented. As discussed previously, the heat shunt can be fabricated from a material having a low coefficient of thermal expansion so that the gap" is a thermal discontinuity rather than an actual perceptible dimensional gap. It is also known, of course, to coat the contacting surfaces of the shell and heat shunt with a substance to prevent corrosion of those surfaces and to inhibit the buildup of foreign matter. Materials with the required properties, such as platinum, iridium, nickel, and their alloys, are well known, as are the surface finishes best suited for the requirements. A physical shield can also be utilized to advantage. Such shield may comprise a thin metal cup shaped like a hollow, truncated cone as shown in F l0. 12. The shield 70 fits inside the insert 29 and may have its lower edge 71 bonded in a gastight relationship to the lower edge of the insert as by resistance welding 72. The upper edge 73 of the shield can be fitted in an annular groove 75 formed in the bottom of the heat shunt 31 and, if the coefficients of thermal expansion of the two elements are suitable, upper edge 73 can be brazed or hard-soldered 74 in the groove 75. Shield 70 should be fabricated of a thin-enough material to allow it to deflect to accommodate for differences in expansion between it and the heat shunt 31 and the insert 29 and the material should be such as to inhibit heat transfer therethrough. A reflective coating of one of the materials discussed above may be used on the inside surface 76 of the shield as a heat reflecting means.

Thus, although shown and described in what are believed to be the most practical and preferred embodiments, it is apparent that departures therefrom will suggest themselves to those skilled in the art and may be made without departing from the spirit and scope of the invention. I, therefore, do not wish to restrict myself to the particular details illustrated and described, but desire to avail myself of all modifications that may fall within the scope of the appended claims.

Having thus described my invention, what I claim is:

l. In a spark plug having a shell with a seating ledge formed on the inner wall surface thereof, an insulator subassembly positioned in a gastight relationship upon said ledge, said insulator subassembly comprising an insulator having a center electrode positioned therein, said insulator having a lower end portion extending below said ledge in a spaced-apart relationship with the surrounding inner wall surface of said shell, a ground electrode on said shell positioned in operative relationship with said center electrode, the improvement for varying automatically the heat range of said spark plug comprising a heat shunt fixed in heat transferring relationship on said insulator end portion and having in one temperature range a thermally nonconducting gap between the shunt outer face and said inner wall surface, said gap having a relationship with the thermal expansion characteristics of said shunt whereby in a second temperature range said gap is closed by the thermal expansion of said shunt to provide a relatively short heat path into said shell such that the temperature of said insulator end portion is thereby modified.

2. A spark plug as defined in claim 1 wherein the heat shunt is a thermally conductive metal element.

3. A spark plug as defined in claim 1 wherein the heat shunt is a thermally conductive metal ring.

4. A spark plug as defined in claim 1 wherein the heat shunt is positioned on the insulator lower end portion intermediate its lower end.

5. A spark plug as defined in claim 1 wherein the face of the outer periphery of the heat shunt is tapered and wherein the mating surface of the shell inner wall surface in operative association with said heat shunt has a matching taper.

6. A spark plug as defined in claim 1 wherein there is more than one heat shunt and wherein the thermal expansion characteristics of said heat shunts are such that the thermally nonconductive gaps between said shunt outer faces and said inner wall surface are selected such that said gaps close in different predetermined temperature ranges to provide relatively different heat paths into said shell such that the temperature of said insulator end portion is thereby variously modified.

7. A spark plug as defined in claim 5 wherein the mating surface in operative association with the heat shunt is a separate annular insert bonded in thermally conductive relationship in the lower inner wall portion of the shell.

8. A method for manufacturing a spark plug of the type having a shell with a seating ledge formed on the inner wall surface thereof, an insulator subassembly having an insulator with upper and lower shoulders and sealing means associated therewith and a lower end portion, a heat shunt fixed in heat transferring relationship on said insulator end portion, the outer peripheral face of said shunt being tapered, the shell inner wall surface in operative association with said shunt having a taper matching said shunt taper, comprising the steps of inserting said insulator subassembly into said shell, heating said insulator subassembly and said shell to a predetermined temperature at which the thermal expansion characteristics of said shunt and said shell have been designed to have expanded said shunt into thermal transfer contact with said shell, positioning said insulator subassembly in said shell with the face of the shunt pressed in thermal transfer contact with said shell inner wall surface, and locking said insulator subassembly in a gastight relationship in said shell.

9. A method for manufacturing spark plugs of the type having a shell with a seating ledge formed on the inner wall surface thereof, an insulator subassembly positioned in a gastight relationship upon said ledge, said insulator subassembly comprising an insulator having a center electrode positioned therein, said insulator having a lower end portion extending below said ledge in a spaced apart relationship with the surrounding inner wall surface of said shell, a heat shunt fixed in heat transferring relationship on said insulator end portion, the outer peripheral face of said shunt and said inner wall surface of said shell in operative association with said shunt face having tapers that match one another, comprising the steps of heating said spark plug to a predetermined temperature at which the thermal expansion of the heat shunting means will have caused the shunt to have expanded into thermal transfer contact with the shell, passing a predetermined level of energy into said heat shunt, measuring the output of energy that has passed through said shunt and into the shell whereby a correlation can be made of the energy flux through the shunting means and the thermal transfer contact thereof, calibrating said monitoring equipment to indicate the correct thermal transfer contact of said shunt as a function of the energy flux therethrough, and subsequently during the positioning of the insulator subassembly in the shell in the manufacture of plugs of this type using the monitoring equipment so calibrated to thereby determine the attainment of correct shunt thermal transfer contact by passing a predetermined level of energy into said heat shunt such that the indication of a certain energy flux level will thus signal the establishment of said correct shunt contact.

10. The method as defined in claim 9 wherein the energy is electromagnetic energy.

11. The method as defined in claim 9 wherein the energy is ultrasonic energy. 

1. In a spark plug having a shell with a seating ledge formed on the inner wall surface Thereof, an insulator subassembly positioned in a gastight relationship upon said ledge, said insulator subassembly comprising an insulator having a center electrode positioned therein, said insulator having a lower end portion extending below said ledge in a spaced-apart relationship with the surrounding inner wall surface of said shell, a ground electrode on said shell positioned in operative relationship with said center electrode, the improvement for varying automatically the heat range of said spark plug comprising a heat shunt fixed in heat transferring relationship on said insulator end portion and having in one temperature range a thermally nonconducting gap between the shunt outer face and said inner wall surface, said gap having a relationship with the thermal expansion characteristics of said shunt whereby in a second temperature range said gap is closed by the thermal expansion of said shunt to provide a relatively short heat path into said shell such that the temperature of said insulator end portion is thereby modified.
 2. A spark plug as defined in claim 1 wherein the heat shunt is a thermally conductive metal element.
 3. A spark plug as defined in claim 1 wherein the heat shunt is a thermally conductive metal ring.
 4. A spark plug as defined in claim 1 wherein the heat shunt is positioned on the insulator lower end portion intermediate its lower end.
 5. A spark plug as defined in claim 1 wherein the face of the outer periphery of the heat shunt is tapered and wherein the mating surface of the shell inner wall surface in operative association with said heat shunt has a matching taper.
 6. A spark plug as defined in claim 1 wherein there is more than one heat shunt and wherein the thermal expansion characteristics of said heat shunts are such that the thermally nonconductive gaps between said shunt outer faces and said inner wall surface are selected such that said gaps close in different predetermined temperature ranges to provide relatively different heat paths into said shell such that the temperature of said insulator end portion is thereby variously modified.
 7. A spark plug as defined in claim 5 wherein the mating surface in operative association with the heat shunt is a separate annular insert bonded in thermally conductive relationship in the lower inner wall portion of the shell.
 8. A method for manufacturing a spark plug of the type having a shell with a seating ledge formed on the inner wall surface thereof, an insulator subassembly having an insulator with upper and lower shoulders and sealing means associated therewith and a lower end portion, a heat shunt fixed in heat transferring relationship on said insulator end portion, the outer peripheral face of said shunt being tapered, the shell inner wall surface in operative association with said shunt having a taper matching said shunt taper, comprising the steps of inserting said insulator subassembly into said shell, heating said insulator subassembly and said shell to a predetermined temperature at which the thermal expansion characteristics of said shunt and said shell have been designed to have expanded said shunt into thermal transfer contact with said shell, positioning said insulator subassembly in said shell with the face of the shunt pressed in thermal transfer contact with said shell inner wall surface, and locking said insulator subassembly in a gastight relationship in said shell.
 9. A method for manufacturing spark plugs of the type having a shell with a seating ledge formed on the inner wall surface thereof, an insulator subassembly positioned in a gastight relationship upon said ledge, said insulator subassembly comprising an insulator having a center electrode positioned therein, said insulator having a lower end portion extending below said ledge in a spaced apart relationship with the surrounding inner wall surface of said shell, a heat shunt fixed in heat transferring relationship on said insulator end portion, the outer peripheral face of said shunt and said inner Wall surface of said shell in operative association with said shunt face having tapers that match one another, comprising the steps of heating said spark plug to a predetermined temperature at which the thermal expansion of the heat shunting means will have caused the shunt to have expanded into thermal transfer contact with the shell, passing a predetermined level of energy into said heat shunt, measuring the output of energy that has passed through said shunt and into the shell whereby a correlation can be made of the energy flux through the shunting means and the thermal transfer contact thereof, calibrating said monitoring equipment to indicate the correct thermal transfer contact of said shunt as a function of the energy flux therethrough, and subsequently during the positioning of the insulator subassembly in the shell in the manufacture of plugs of this type using the monitoring equipment so calibrated to thereby determine the attainment of correct shunt thermal transfer contact by passing a predetermined level of energy into said heat shunt such that the indication of a certain energy flux level will thus signal the establishment of said correct shunt contact.
 10. The method as defined in claim 9 wherein the energy is electromagnetic energy.
 11. The method as defined in claim 9 wherein the energy is ultrasonic energy. 